1. Field of the Present Invention
The present invention relates to a stabilized optical pulse generator, and more particularly the rational harmonically mode-locked fiber ring laser (RHML-FRL) of a stabilized optical pulse generator. Optical pulse trains generated from the RHML-FRL are stabilized to a high degree with the present invention. The RHML-FRL incorporating invention, can be used as an optical pulse generating source for a high-speed optical communication system, or an optical time-division multiplexing (OTDM) system, etc., which needs stabilized optical pulse trains at high repetitions frequency. Further, the present invention can also stabilize the optical pulse train at a repetition frequency in microwave frequency band, millimetre-wave frequency band, and higher frequency bands than the frequencies, mentioned in the invention.
2. Prior Art of the Present Invention
Short optical pulses at high repetition rates are important for an ultra high-speed optical communication system. Presently, an active harmonically mode-locked fiber ring laser (ML-FRL) is very popular because of its ability to generate short and transform-limited optical pulses at high repetition rates. When RF modulation signal which is a harmonic of the cavity resonance frequency is applied to the RF port of the Mach-Zehnder intensity modulator (MZM) placed inside the cavity and biased at the quadrature point on its transmission characteristic curve (see FIG. 2), optical pulses at a repetition frequency equal to the applied RF modulation signal frequency are generated. However in such lasers, the maximum pulse repetition frequency is limited by the modulator bandwidth as well as frequency of the drive electronics. Amongst many demonstrated techniques to increase the pulse repetition (PRF) in ML-FRLs, rational harmonic mode-locking has become gradually popular because of the advantage that the PRF can be increased by simple detuning of the modulation signal frequency (see Z. Ahmed, and N. Onodera, “High-repetition rate optical pulse generation by frequency multiplication in actively mode-locked fiber ring lasers,” Electron. Lett., vol. 32, pp. 455-457, 1996.)
FIG. 14A shows a prior art of the rational harmonically mode-locked fiber ring laser (RHML-FRL). In FIG. 14A, a reference symbol A is an optical amplifier, which comprises an excitation light source, and a gain medium which has optical gain characteristics. Reference numeral 1 is a gain medium which is erbium doped fibers, that is Er/Yb doped fibers. Reference numeral 2 is an excitation light source. Reference numeral 3 is a coupler (optical coupler). Reference numeral 4 is a modulator which is a Mach-Zehnder optical intensity modulator. Each of reference numerals 5, 6 and 7 is an optical isolator. Reference numeral 8 is a polarisation controller (PC). Reference numeral 9 is an optical coupler, which branches the oscillation light of the optical fiber ring to an optical sensor 23 at the ratio 90:10 (the excitation light in the optical fiber of 10% is branched to the optical sensor). Reference numeral 12 is optical fibers. Reference numeral 21 is electric oscillator which generates high frequency signals. Reference numeral 22 is an electric amplifier. Reference numeral 23 is an optical sensor which converts an optical signal to an electric signal. Reference numeral 24 is a measuring instrument which measures the signal converted to the electric signal.
In the system of FIG. 14A, the laser light generated in the excitation light source 2 excites the Erbium fibers (EDF) that is a gain medium, and the optical fiber ring excites the optical laser light of frequency of the resonance frequency fc and integer times of the resonance frequency fc. The electric oscillator 21 can generate electric signals of frequency fm of integer times of fc, and generates further the frequency of fc/p, which is due to the applied modulation frequency that is a little shifted from the original electric signal of frequency fm (in the followings, applying the frequency a little shifted is called “detuning”). The modulator 4 is biased with a bias voltage Vb.
In the system of FIG. 14A, the modulator 4 generates optical pulses of high frequencies of even times or odd times of the applied modulation frequency fm in the optical fiber ring according to the applied voltage Vb. When the frequency applied to the modulator is detuned by fc/p from fm, optical pulse trains of repeating frequency (PRF) of p(fm±fc/p) is generated as shown in FIG. 14B. The action of the rational harmonic mode-locking of FIG. 14A is explained referring FIGS. 1A-1E.
FIGS. 1A-1E are explanation drawings of time domain depiction of pulse repetition frequency multiplication in the RHML-FRL. In FIG. 1, trace 1 shows loss in the cavity while pulse repetition frequency multiplication is shown in trace 2-trace 5 in FIG. 1
The pulse repetition frequency multiplication via rational harmonic mode-locking is based on an interaction between circulating optical pulses and loss modulation within the laser cavity. In active harmonically mode-locked fiber ring laser, the laser is mode-locked at the nth harmonic of its cavity resonance frequency and there are n-number of pulses circulating inside the cavity and on every cavity round-trip, each pulse passes through the modulator at minimum loss or maximum transmission point in its modulator transfer characteristic curve as shown in trace 2 in FIG. 1. When the applied modulation signal frequency is decreased by f/p, there is corresponding time delay of T/p in the arrival of each circulating pulse with respect to loss minima, where T is the modulation period.
For example, when fm is decreased by fc/2, each pulse is delayed by half the modulation period i.e. T/2. As a result, each pulse on its next arrival at the modulator will pass through the modulator at loss maxima as shown in the trace 3. Here pulse ‘A’ which was originally at a point of loss minima ‘0’ arrives at a point of loss maxima which is at half-way between loss minima ‘2’ and ‘3’ after one cavity round-trip, hence receiving maximum loss. Since relaxation time of the EDF-gain medium is much longer (of the order of 10 milliseconds) compared to pulse round-trip time (of the order of tens of microseconds), a complete suppression of pulse ‘A’ does not occur. However during 2nd cavity round-trip, pulse ‘A’ is further delayed by T/2 such that it now arrives at loss minima ‘4’ and therefore receives minimum loss or maximum gain. It means that each pulse will experience minimum loss or maximum gain on every 2nd round-trip of cavity which is also true for other pulses circulating within the cavity. As a result, pulse repetition rate of output optical pulse trains would now be twice the repetition rate realised via conventional harmonic mode-locking, thereby establishing pulse repetition frequency doubling. The pulse repetition frequency tripling and quadrupling can also be explained in the same manner and are shown in traces 4 and 5, respectively. In the case of pulse repetition frequency quadrupling, the fm is detuned by fc/4 leading to each pulse delayed by T/4 and pulse ‘A’ in the pulse sequence ‘ABCD’ would now experience minimum loss once in every 4th cavity round-trips which leads to pulse repetition frequency quadrupling as shown in trace 5 in FIG. 1E. In the following explanation, fm′ is defined as fm′=fm±fc/p.
There are several advantages of RHML-FRL such as an optical pulse train can be generated at a much higher repetition rates by using lower frequency drive electronics. However the main limitation associated with RHML-FRL is that the laser suffers with the inherent pulse amplitude instability which includes both amplitude noise and inequality in pulse amplitudes. The amplitude noise in the generated optical pulses is mainly due to supermode noise caused by the unequal energy distribution amongst the cavity modes which is attributed to the longer relaxation time of the gain medium compared to the pulse round trip time. While the unequal pulse amplitudes in optical pulse trains are mainly due to asymmetric cavity loss modulation within the cavity caused by the presence of randomly oscillating intermediate modes that are frequency spaced at fm, 2 fm, 3 fm (for PRF=4fm) and become prominent at a pulse repetition frequency greater than 2 fm (for p>2). As a result, in RHML-FRL, the optical pulse trains with repetition frequency greater than 2 fm exhibit high degree of pulse amplitude instability.
In order to increase pulse amplitude stability in RHML-FRL, it is very important that the randomly oscillating cavity resonance modes at fc, and intermediate longitudinal modes at fm, 2 fm, etc. are suppressed relative to the main oscillating longitudinal modes which are phase matched with each other and separated by frequency pfm which is also equal to the desired pulse repetition frequency.
To make RHML-FRL as an useful optical pulse source for practical applications in high-speed optical communications networks, it is essential that the generated pulses are equal in amplitude with minimum amplitude noise.
In such lasers, only one attempt has been made to increase pulse amplitude stability (see M. Y. Jeon, H. K. Lee, J. T. Ahn, D. S. Lim, H. Y. Kim, K. H. Kim, and E. H. Lee, “External fiber laser based pulse amplitude equalisation scheme for rational harmonic mode-locking in a ring-type fiber laser,” Electron., Lett., vol 34, pp. 182-184, 1998.)
FIG. 15 shows the system of M. Y. Jeon etc. In FIG. 15, reference numeral 31 is a RHML-FRL Reference numeral 32 is PML-FRL (passively mode-locked fiber ring laser). Reference numeral 33 is NALM (non-liner amplifying loop mirror). Reference numeral 34 is a Faraday rotating mirror. Reference numeral 35 is a polarisation controller.
In the system of FIG. 15, the pulse amplitude equalisation of optical pulse train at a repetition rate of 4 fm (for p=4) was achieved by a complex method. In this technique, pulse amplitude equalisation was based on the additive pulse mode-locking (APM) action within the externally coupled passively mode-locked fiber ring laser (PML-FRL) which consists of a nonlinear amplifying loop mirror (NALM), and a linear Faraday rotating mirror. The APM seems to be as a non-linear amplitude modulation produced by interfering two self-phase modulated versions of the same mode travelling within the PML-FRL. The combination of polarisation controller and the Faraday rotating mirror transform linear polarisation into elliptic polarisation which then rotates via the Kerr-effect of optical fiber, the angle of rotation being proportional to intensity of the circulating optical signal. This polarisation based on the intensity discrimination then leads to pulse amplitude equalisation of optical pulses from RHML-FRL.
It is evident that this reported technique of pulse amplitude equalisation in an RHML-FRL is very complex to realise and works under certain very stringent operating conditions such as the cavity resonance of the externally coupled PML-FRL must be exactly equal to an harmonic multiple of the resonance frequency of the main RHML-FRL cavity, requires delicate adjustments of the polarisation controller as well as the NALM loop length. Besides that many additional components such as optical erbium-doped fiber amplifier, polarisation controller, Faraday rotating mirror, optical couplers, isolators etc., are required to assemble the PML-FRL and therefore such a scheme becomes very expensive to realise. Further more, extra cavity structure not only increases the overall size of the complete laser system, but also quite sensitive to the surrounding temperature variations and mechanical disturbances which can easily disturb the requirement of the perfect matching of resonance frequency of PML-FRL with that of the RHML-FRL and thereby making this scheme a very complex and difficult to realise.